There are a number of conventional processes for packaging integrated circuits. One approach which is commonly referred to as “flip chip” packaging generally contemplates forming solder bumps (or other suitable contacts) directly on an integrated circuit die. The die is then “flipped” and attached to a substrate such as a printed circuit board or substrate. That is, the solder bumps on the die are aligned and mounted onto matching contacts on the substrate. The solder bumps are then reflowed to electrically connect the die to the substrate.
When a flip chip die is mounted to the substrate, an air gap typically remains between the die and substrate. This gap is commonly filled with material that is flowed into the gap in liquid form and is then solidified. This material is generally a mixture of a epoxy resin and small silica spheres and is often called underfill. The underfill material is typically applied in liquid form from a dispenser at one edge of the die. The material then flows into the narrow gap due to capillary action and spreads across the flip chip die until finally the entire area of the gap between the die and substrate is filled.
There are problems associated with this type of underfill process. For example, the operation of applying underfill must be repeated for each flip chip mounted onto a substrate. Repeating such an operation many times during manufacturing significantly increases costs. Also, as the underfill material flows past solder bumps to fill the gap, separation of silica spheres from resin may occur. The separation of silica spheres from the resin occurs as some silica spheres become trapped as they meet solder ball obstacles. The underfill material may therefore develop streaks of high and low silica concentration. The silica may also separate from the resin by sinking to one side of the gap, thus creating a silica rich side in the bottom and a resin rich side on the top of the gap. This segregation of silica and resin alters the mechanical properties of the filled region and thereby may mitigate the mechanical function of the underfill.
Although the described process works well, there is always a desire to provide more cost effective processes for packaging integrated circuits.